Which of the following statements about calcium signaling in muscle is true?

• A. Calcium directly increases mTOR activity
• B. Calcium suppresses AMPK
• C. Calcium can activate calmodulin-dependent kinase
• D. Calcium reduces glucose uptake

Answer: (C)

Calcium signals in muscle act by binding to calmodulin, forming a calcium-calmodulin complex that directly activates calmodulin-dependent kinases (CaMKs), especially CaMKII. This activation links the electrical signal of muscle contraction to metabolic and gene-regulatory processes, guiding how the muscle responds to activity. Because CaMKs phosphorylate various targets involved in metabolism, gene expression, and contraction, calcium’s direct effect is to drive CaMK activity rather than act directly on other pathways.

In other words, calcium’s primary direct action here is to turn on CaMK enzymes. The idea that calcium directly boosts mTOR isn’t correct, since mTOR activation is driven more by nutrients and growth factors (with CaMK signaling able to influence it indirectly). Calcium does not suppress AMPK; in fact, calcium can activate AMPK through CaMKKβ, which helps promote glucose uptake and energy balance during activity. And rather than reducing glucose uptake, calcium signaling during contraction supports GLUT4 translocation and increased glucose uptake to meet energy demands.